1. Field of the Invention
The present invention relates to thermosetting powder coating compositions and curing agent compositions, and more particularly to thermosetting powder coating compositions excellent in storage stability and capable of providing coating films with excellent appearance and also to curing agent compositions for use in the thermosetting powder coating compositions.
2. Description of Related Art
Powder coatings which do not release organic solvents to an atmosphere have been widely noted as eco-friendly coatings.
Currently, the use of thermosetting powder coatings is popular in the powder coating field for their ability to provide coating films excellent in performances and physical properties. For their application to automotive bodies, the improvements in smoothness and appearance of resulting films have been demanded. One technique used to improve the film smoothness is the thick application of powder coatings. Another technique is the use of powder coatings prepared from the materials having low melt viscosity. While effective in achieving some improvements in smoothness and appearance of resulting films on automobile horizontal portions such as a hood and the like, these techniques have suffered from the problem. That is, the powder coatings applied onto automobile vertical portions such as doors and the like, when subsequently heated and melted, result in coating films which show a marked reduction in smoothness and appearance as a result of the occurrence of sagging.
The incorporation of fine crosslinked resin particles, called microgels, to prevent the occurrence of sagging is known in the solvent-borne coating field. For example, Japanese Patent Laying-Open Nos. Sho 49-97026 (1974) and Sho 60-250068 (1985) disclose techniques which contemplate to reduce sagging and accordingly improve film appearance by adding microgels to top coatings.
In the powder coating field, the use of powder coatings containing fine crosslinked resin particles is disclosed, for example, in Japanese Patent Laying-Open Nos. Sho 62-97026 (1987). However, such powder coatings are prepared by mixing dry fine particles of crosslinked resin under a solvent-free condition. During the mixing, the crosslinked resin particles are often caused to locally form agglomerates. This disturbs uniform distribution thereof to result in the unsatisfactory smoothness and appearance of obtaining coating films. In the case where a melt mixing process is added to precede the dry mixing process, a majority of fine particles of crosslinked resin are caused to deposit on surfaces of resulting powder coating particles. When such powder coating particles are applied and subsequently heated, the interaction is caused to occur between the fine crosslinked resin particles. This has led often to the insufficient smoothness and appearance of resulting coating films.
Powder coatings, when heated to melt, result in coating films. However, such coating films show the insufficient smoothness compared to those obtained from solvent-borne coatings. In order to improve the smoothness of resulting coating films, the melt viscosities of components contained in the powder coatings, i.e., the melt viscosity of raw material must be reduced to improve the fluidity thereof during melt.
One approach to lower the melt viscosity of raw material may be the use of low-melting point or low-molecular-weight substances for the raw material. While effective in improving the smoothness of resulting films, their use has also led to the reduction in storage properties, such as resistance to blocking and solid reaction. The difficulty has thus been to reconcile the storage stability of the coating composition and the smoothness of resulting film.
In Japanese Patent Laying-Open No. Hei 9-100414 (1997), a method is disclosed which produces thermosetting resin particles having a narrow particle size distribution by utilizing a wet process. Since the thermosetting resin particles are produced in an aqueous medium, their use has led to the improvement in one of storage properties, i.e., resistance to solid reaction. It has been unsatisfactory, however, to reconcile the blocking resistance of the powder coating composition and the smoothness of the obtaining film.
Among thermosetting powder coatings, acrylic-based coatings, when formed into films, provide excellent performances and physical properties. The resulting films however suffer from a problem of poor appearance, called a hazing phenomenon. This phenomenon is known to often occur when a large amount of polybasic acid is used as a curing agent. If the reduced amount of polybasic acid is used to suppress the occurrence of such a hazing phenomenon, the performances and physical properties of resulting films then become insufficient. Also, there exists no polybasic acid which can act to improve performances and physical properties of resulting films while controlling the occurrence of the hazing phenomenon, so far as we know.
It is a first object of the present invention to provide a thermosetting powder coating composition which, when applied and then heated, shows no occurrence of sagging to result in coating films with satisfactory smoothness and appearance.
It is a second object of the present invention to provide a thermosetting powder coating composition which has an excellent storage stability and can be formed into films with excellent smoothness.
It is a third object of the present invention to provide a curing agent composition which, when used as a curing agent for acrylic based powder coatings, serves to provide films which show no occurrence of hazing phenomenon, and also to provide a, method of producing the curing agent composition.
In accordance with a first aspect of the invention, a thermosetting powder coating composition is provided which is prepared by a wet process from a formulation containing an epoxy-containing acrylic resin (a), a polycarboxylic acid compound curing agent (b) and fine crosslinked resin particles (c).
The above-stated wet process preferably includes the following steps:
1. The above-described formulation components are allowed to dissolve or disperse in an organic solvent to provide a raw material solution.
2. The raw material solution obtained in step 1 is added to an aqueous solution containing a water-soluble polymer that has a cloud point within the temperature range of 30-90xc2x0 C. and mix them at a temperature below the cloud point to thereby prepare a suspension containing primary oil particles.
3. The suspension obtained in step 2 was heated to a temperature equal to or above the cloud point to form secondary oil particles and the organic solvent was distilled off to collect particles.
The aforementioned epoxy-containing acrylic resin (a) contains an epoxy-containing acrylic resin A and an epoxy-containing acrylic resin B. These acrylic resins A and B preferably satisfy the following relationships:
(1) (SPA-SPB) is within the range of 0.2-1.5, where SPA=solubility parameter of resin A and SPB=solubility parameter of resin B;
(2) Tg(A)-Tg(B)xe2x89xa710xc2x0 C., where Tg(A)=glass transition temperature of resin A and Tg(B)=glass transition temperature of resin B;
(3) Tg(A) is within the range of 40-100xc2x0 C. and Tg(B) is within the range of 20-50xc2x0 C.; and
(4) A ratio in solid weight of resin A to B is within the range of 5/95-50/50.
The aforementioned formulation may further contain a resin (d) that exists in the liquid form at room temperature and preferably satisfies the following relationships:
(1) (SPB-SPd) is within the range of 0.01-1.5, where SPd is solubility parameter of resin (d); and
(2) A solids weight of the resin (d) is 5-70 parts, based on 100 parts of a total solids weight of the epoxy-containing acrylic resin (a) and resin (d).
In accordance with a second aspect of the present invention, a thermosetting powder coating composition is provided which is prepared by a wet process from a formulation containing an epoxy-containing acrylic resin (a), a polycarboxylic acid compound curing agent (b) and a resin (d) that exists in the liquid form at room temperature.
The above-stated wet process preferably includes the following steps:
1. A raw material solution is formed by allowing the above-described formulation to dissolve or disperse in an organic solvent.
2. The raw material solution obtained in step 1 is mixed with an aqueous solution containing a water-soluble polymer which shows a cloud point at a temperature within the range of 30-90xc2x0 C. to prepare a suspension containing primary oil particles.
3. The suspension obtained in step 2 was heated to a temperature of not below the cloud point to prepare secondary oil particles while removing the organic solvent to collect particles.
The aforementioned epoxy-containing acrylic resin (a) may contain an epoxy-containing acrylic resin A and another epoxy-containing acrylic resin B. The formulation components preferably satisfies the following relationships:
(1) A number average molecular weight of resin A is in the range of 2,000-4,000;
(2) Tg(A) is within the range of 40-100xc2x0 C., where Tg(A)=glass transition temperature of resin A;
(3) Tg(A)-Tg(B)xe2x89xa710xc2x0 C., where Tg(B)=glass transition temperature of resin B;
(4) (SPA-SPB) is within the range of 0.2-1.5, where SPA=solubility parameter of resin A and SPB=solubility parameter of resin B;
(5) A solids amount of resin B is 50-95% by weight of the total solids of epoxy-containing acrylic resin (a);
(6) (SPB-SPd) is within the range of 0.01-1.5, where SPd=solubility parameter of resin (d); and
(7) The solids amount of resin (d) is 5-70 parts by weight, based on 100 parts by weight of the total solids of the aforementioned epoxy-containing acrylic resin (a) and resin (d).
In accordance with a third aspect of the invention, a curing agent composition (b) is provided which contains a polycarboxylic acid compound (b-1) that exists in the crystalline solid form at room temperature, and a carboxylic acid compound (b-2) dissimilar in type to the polycarboxylic acid compound (b-1). Characteristically, the curing agent composition satisfies the following relationships:
(1) The curing agent composition (b) exists in the solid form at room temperature; and
(2) A melting point of the curing agent composition (b), as determined by DSC (differential scanning calorimeter), is lower than a melting point of the polycarboxylic acid compound (b-1) or the carboxylic acid compound (b-2).
The melting point of the curing agent composition (b) is preferably within the 60-180xc2x0 C. range.
Preferably, the melting point of the curing agent composition (b) is lower than the melting point of the polycarboxylic acid compound (b-1) and lower than the melting point of the carboxylic acid compound (b-2).
Preferably, a ratio in weight of the polycarboxylic acid compound (b-1) to carboxylic acid compound (b-2) is 50:50-99:1.
A multilayer film-forming method of the present invention includes the steps of:
applying a basecoat onto an undercoated and optionally intercoated substrate;
applying the powder coating composition of the present invention onto the basecoat; and
heating the substrate carrying thereon the basecoat and powder coating layers.
A multilayer film of the present invention is the multilayer film prepared according to the above-described multilayer film-forming method.